A numerical control device (referred to as an NC device) moves a tool relative to a workpiece on the basis of previously created NC data to subject the workpiece to numerically controlled machining as commanded. In such numerical control, the trajectory of the tool center does not coincide with the desired machined profile, that is, with the path specified by the NC data, but rather is a path offset, by an amount equivalent to the tool diameter, from the machined profile to the right or left side thereof with respect to the travelling direction. Such a tool center trajectory is calculated automatically by the NC device if the machined profile and tool diameter, as well as a G-instruction for executing the tool diameter offset, are given. The tool is transported along the obtained path of the tool center to finish the workpiece into the desired machined profile. It should be noted that there are also cases where NC data indicative of the path of the tool center (which data are referred to as offset NC data) are created from the abovementioned NC data and tool diameter, and the tool is moved in accordance with the offset NC data to subject the workpiece to the desired machining.
There are situations where, depending upon the machined profile (part profile) and tool diameter, a tool with interfere with the workpiece when the tool is transported along the tool center path obtained by the offset, thereby resulting in damage to the tool or in excessive cutting which makes it impossible to obtain a part having the commanded profile.
It is therefore necessary to check beforehand whether the tool will interfere with the workpiece when cutting is performed while the tool is being offset in accordance with created NC data.
However, a method of checking for tool interference in a simple manner does not exist in the prior art.